A lot of attempts are being made to develop a technology for converting biomass for commercial production of bioethanol as a renewable energy source. Worldwide, ethanol is mostly produced by fermentation, accounting for up to 90% of ethanol produced globally. Although most bioethanol is produced from sugar cane (Brazil) or molasses and corn (the United States), other starch materials such as wheat, barley and rye are also suitable to be used. The starch-containing grains have to be converted into sugar. In Europe, starch grains (e.g., wheat) and sugar beet are the major source of bioethanol production.
The fermentation of sugar into ethanol is a large-scale technology that has been developed for commercial purposes. The fermentation technology has been developed and optimized for thousands of years. The cost for the fermentation is largely dependent on the price of the biomass source and may account for 55-80% of the final price of ethanol. Brazil and the United States produce the majority of bioethanol consumed globally, and the global demand on bioethanol is increasing abruptly. This inevitably increases the price of corn, sugar cane and other important grains.
The starch-containing grain is subjected as milling or grinding in order to release starch. Then, it is diluted in water and then cooked to dissociate all water-soluble starches. At the same time, the starch is converted into sugar. This process may be carried out by enzymatic or acid hydrolysis. In acid hydrolysis, diluted mineral acid is added to the grain slurry before cooking. The resultant short carbohydrates may be fermented by microorganisms such as yeast. Ethanol is produced through the fermentation, and the ethanol may be concentrated through a series of distillation and dehydration procedures. Sugar is converted to ethanol as a result of the fermentation process.
Other plant sources may also be a good source of sugar. Actually, the selection of the plant source depends not only on what plant grows well in the particular region but also on the sugar content and availability of the plant. New promising techniques will allow more practicable production of ethanol. The plants sources for producing ethanol from the stem, roots or leaves of plants rather than from the kernel of corn are known as cellulosic material. The so-called cellulose ethanol has been known from several years ago, but it is regarded as ineffective and expensive to break down cellulose by fermentation. The recent technical advancement as well as high oil price provides a more competitive edge. However, many people still agree that more progress in technology is required to reduce the current production cost.
Plant cell walls are composed of lignocellulosic materials, which are represented by cellulose (linear glucose polymers), hemicellulose (highly branched heteropolymers) and lignin (crosslinked aromatic macromolecules with large molecular weight). The bonding between the polysaccharide components (cellulose and hemicellulose) and non-polysaccharide components (lignin) is the main cause of mechanical and biological resistance. Cellulose, the most abundant polysaccharide on earth, is a polymer accounting for 50% or more of the wood weight wherein cellobiose (D-glucopyranosyl-β-1,4-D-glucopyranose) is arranged in good order. The cellulose chain which forms fibrils consists of about 10,000 glucose units. The cellulosic material has a crystal domain separated from the less-ordered, amorphous domain, which allows chemical and biochemical attack. Cellulases can hydrolyze the cellulose polymer to monomers, and the resulting glucose is fermented into ethanol by the yeast Saccharomyces cerevisiae. Accordingly, the biocatalysis is at the center of the biomass ethanol technology. Hemicellulose consists of cellulose and lignin. Hemicellulose in wood is a short (100-200 sugar units), highly-branched heteropolymer consisting of the predominant xylose as well as glucose, mannose, galactose, arabinose and other uronic acids. C5 and C6 sugars are linked by 1,3-, 1,6- or 1,4-glucosidic linkages, which differentiate cellulose from lignin, and are often acetylated. Lignin is a 3-dimensional polyphenolic network of dimethoxylated, monomethoxylated and non-methoxylated phenylpropanoid units, derived from p-hydroxycinnamyl alcohol. Lignin is hydrophobic and highly resistant to chemical and biological degradation. Cellulosic fibrils are embedded in an amorphous matrix network of hemicellulose and lignin, and they serve as glues between the plant cells, providing resistance to biodegradation. Other non-structural components (phenols, tannins, fats, sterols, sugars, starches, proteins and ashes) of the plant tissue generally accounts for 5% or less of the dry weight of wood.
In order to hydrolyze the biomass polysaccharides into fermentable sugars, for example by depolymerization, such pretreatment processes as steam explosion, mild acid treatment, strong acid treatment, ammonia treatment, hydroxide treatment, etc. are employed. No matter what it is, the pretreatment process should be environment-friendly and economically feasible. The pretreatment method will be selected considering process dependency and cost, as well as process yield and production parameters.
The present disclosure presents a method for producing new renewable energy allowing to cope with the global climate change by removing carbon dioxide from the atmosphere by photosynthesis, without affecting the global grain prices. And, the by-product produced during bioethanol production may be used as livestock feed additives, fuels for steam and power generation, raw materials of gypsum board, cement additives, fertilizers, or the like. Thus, the present disclosure provides a method for utilizing non-food biomass from corn stover, rice straw, wheat straw, fruit skin, sugar cane stalk or sorghum stalk, which are available in large scale at low cost, 100% as a valuable resource.
Throughout the specification, a number of publications and patent documents are referred to and cited. The disclosure of the cited publications and patent documents is incorporated herein by reference in its entirety to more clearly describe the state of the related art and the present disclosure.